New members

All of identity's members are defined in the
Adaptable Unary Function
requirements. Identity does not introduce any new members.

Notes

[1]
It is essential that the return type and the argument type are
the same: generalizing identity to allow them to differ would not
work. The reason is that identity returns a const reference to
its argument, rather than a copy of its argument. If identity were
allowed to perform a conversion, then this would be a dangling reference.